The invention relates to an earthquake alert system having two or more sensors for detecting tremor signals, and having means for electrical signal processing for processing and analyzing the tremor signals.
It is known that the fact that shock waves spread out through the earth's crust in a characteristic manner can be utilized to detect immediately imminent earthquakes. Different types of waves spread out in all directions from the epicenter of an earthquake. In this connection, a differentiation is made between primary waves (abbreviation: P waves), and secondary waves (abbreviation: S waves). The P waves are compression waves that spread at a clearly greater velocity than the S waves. The S waves are shear waves, which have a significantly greater amplitude than the P waves. The destruction that occurs during earthquakes is caused exclusively by the S waves.
The P waves, which have not only a lower amplitude but also a different vibration spectrum than the S waves, do not cause any destruction and are generally not even perceived by humans. Because of the greater velocity of spread of the P waves, these arrive at a location distant from the epicenter of the earthquake sooner than the destructive S waves. Therefore, the detection of P waves can be utilized to generate an alert before the S wave arrives. The early warning time, in other words the time span between arrival of the P wave and the arrival of the subsequent S wave, depends primarily on the distance from the epicenter of the earthquake.
It is furthermore known that there are relationships between the characteristics of the P waves and the S waves of an earthquake, on the basis of geophysical laws. For this reason, the amplitude of the subsequent S wave and therefore the destructive effect can be predicted or at least estimated from the spectrum and the amplitude of a P wave that is detected.
Every year, seismologists record approximately 20,000 earthquakes. Millions of people live in areas that are at risk of earthquakes. Despite improved earthquake-proof methods of construction, people are always injured or killed in severe earthquakes. This is particularly due to the fact that these people are in buildings that collapse due to the tremors, at the time that the S wave of the earthquake arrives. The great numbers of victims of severe earthquakes are furthermore attributable, to a significant proportion, to fires that are caused by gas lines that ruptured during the earthquake and/or damaged power lines.
It is immediately evident that there is an urgent need for reliably functioning earthquake alert systems. Using such earthquake alert systems, people are supposed to be given the opportunity to leave buildings, or at least go to safe areas within buildings, when a potentially hazardous earthquake is immediately imminent. Modem earthquake warning systems that are able to detect a P wave of an earthquake are suitable for giving an alarm with a sufficient early warning time so that the numbers of victims of severe earthquakes can be significantly reduced.
An earthquake alert system of the type stated initially is known, for example, from WO 99/09433 A1. The previously known system is suitable for detecting upcoming earthquakes, in that the P wave that precedes a potentially destructive S wave of an earthquake is recorded and analyzed, by means of suitable tremor sensors and signal processing means. It is known that sensitive tremor sensors are required for reliable detection of P waves of an earthquake. Because of the required great sensitivity, earthquake warning devices whose function is based on the detection of P waves are susceptible to false alarms. Consequently, there is a conflict of goals, which is difficult to resolve, between reliable and sufficient detection of P waves, on the one hand, and equally reliable avoidance of false alarms, on the other hand. Particularly in the case of earthquake alert systems whose tremor sensors are affixed to buildings in the vicinity of roads and railroad lines, there is the problem that ground vibrations caused by road or railroad traffic are difficult to differentiate from P waves of an earthquake. In order to solve this problem, the aforementioned WO 99/09433 A1 proposes to affix a plurality of tremor sensors at different locations of a building. The tremor signals detected by the individual sensors are transmitted to a central signal processing device of the previously known earthquake alert system by way of suitable (including wireless) transmission lines. This central signal processing device evaluates the tremor signals that arrive from the various sensors. If it is determined, in the analysis of the tremor signals, that two or more sensors for P waves have detected characteristic tremor signals with a time overlap, an alarm is triggered. In other words, in the case of the previously known earthquake alert system, an attempt is made to avoid false alarms in that an alarm is triggered only if P waves are recorded, in agreement, in tremor sensors that are affixed at different locations. In this way, it is effectively avoided that a locally occurring ground vibration that is triggered by a heavy vehicle passing by, for example, is recorded as a P wave of an earthquake and results in an alarm being triggered. In order to ensure that a reliable distinction can be made between P waves of an earthquake and normal, unimportant ground vibrations, without having to accept restrictions with regard to the sensitivity of the earthquake alert system, the aforementioned WO 99/09433 A1 furthermore proposes to store natural tremor patterns typical for the location in a data memory of the central processing unit. In the case of the previously known system, an alarm is given only if tremors whose spectrum deviates from the stored spectrum are recorded. In the case of the previously known earthquake alert system, it is a disadvantage that it is not very flexible in use. Another disadvantage is that the function of the previously known system is impaired, as a whole, if only partial components of the system fail.
The previously known system consists, as already explained, of a central signal processing unit to which a plurality of sensors can be connected. The central processing unit has a separate input connection for every tremor sensor that can be connected. It is therefore disadvantageous that the number of sensors that can be connected is maximally equal to the input connectors present on the central signal processing unit. Accordingly, the previously known system can only be expanded within certain limits. However, it is particularly problematical that the function of the previously known system is restricted if even only a few of the sensors fail. Since the functions that are essential for giving an alarm are brought together in the central signal processing unit, the earthquake alert system as a whole is put out of operation in case of a defect of the central processing system. Due to a lack of redundancy, the reliability of the previously known system and therefore the safety with regard to an earthquake alert is not satisfactory.
Proceeding from this, it is the task of the invention to make available an earthquake alert system that is flexible in use. In this connection, it is supposed to be possible to monitor very different types of constructions (e.g. buildings, bridges, tunnels, roads, sewer systems, etc.) with the lowest possible expenditure of costs.
Furthermore, a redundant and therefore reliable and safe earthquake alert is supposed to be made possible.